1. Field of the Invention
The invention relates to a decorative sheet and/or a molding, in particular for indoor or outdoor applications on buildings, made from a monolaterally or bilaterally decorative-layer-laminated, pressed single- or multilap core layer made from wood fibers and/or from cellulose fibers, or from timber sawdust products, where the fibers or the timber sawdust products have been impregnated with a resin as binder and are surrounded by the hot-curing binder, and also to its use, and to a process for producing a sheet of this type.
2. Description of the Prior Art
The prior art discloses sheets made from woodchips and from wood fibers with a matrix of amino plastic or of phenolic resins or cement, and also discloses plywood sheets, where these have uniform density and are suitable for external applications or internal applications in damp conditions. However, unlike high-pressure laminate sheets, sheets of this type are not maintenance-free, and they generally require post-treatment on all sides due to severe water absorption through the nonsealed surface. Water absorption can cause severe swelling, with an increase in sheet thickness. Sheets of this type have low strength and are not decorative. The core is usually composed of solid sheets of soda kraft paper and of a phenolic resin, without a decorative layer, or with a decorative layer on one or both surfaces of the core. Hot-curing resins are cured at high pressure and high temperature, and together with the sodium kraft paper form a solid homogeneous, integrated sheet. Sheets of this type are termed HPL sheets or high-pressure laminate sheets.
U.S. Pat. No. 3,673,020 (DE-A 19 12 300) describes a decorative sheet molding made from sawdust and comprising a matrix of phenolic resin, the resin content being from 5 to 15% by weight, based on the dry weight of the sawdust. The weathering resistance of this molding is unsatisfactory. In weathering tests there is marked absorption of water with associated edge swelling and splitting of the pressed core, causing surface cracking after just a few weeks.
EP-B 0 081 147 (U.S. Pat. No. 4,503,115) discloses a decorative panel composed of a pressed core made from wood particles surrounded by hot-curing phenol-formaldehyde resin. On one or both surfaces of the core of the panel there are decorative layers composed of a woven or nonwoven fabric or cloth or of a plastics film or paper foil or wood foil, and/or of a coating layer. The wood fibers and/or cellulose fibers of the core have a maximum length of 20 mm, and the wood fibers here have been coated with the hot-curing phenol-formaldehyde resin in aqueous solution or dispersion. The resin content is more than 150 g per 1 000 g of dry fibers, and extends to 900 g. The water content of the wood fibers is of the order of from 3 to 10% by weight, and the density of the panel is from 1 100 to 1 500 kg/m3. In particular, the decorative layer is composed of a melamine-formaldehyde-resin-saturated decorative film whose weight per unit area is from 80 to 400 g/m3. Where appropriate, the decorative layer comprises a filler and/or a dye. If the surface of the pressed core is decorated by coating with a crosslinked acrylic, urethane, epoxy, or melamine resin, this coating comprises, where appropriate, a filler and/or a dye.
The manner of producing this decorative panel is that one or more mechanically precompacted layers based on wood fibers and laid one on top of the other are hot-press-molded to give the core, whereupon the hot-curing phenol-formaldehyde resin of the precompacted layers cures. One or both surfaces of the core are equipped with a decorative layer, which is either applied to the precompacted mat made from a number of layers, or is applied to the core once it has been produced by press-molding this mat.
This known panel has very uniform dimensional stability in longitudinal and transverse directions and is highly dimensionally stable even when exposed to severe weathering cycles. When comparison is made with other known sheets made from wood particles and from wood fibers or cellulose fibers, this panel has higher flexural strength and higher tensile and transverse strength, higher dimensional stability, and reduced water absorption. This panel is a laminate made from pre-press-molded wood-fiber-resin layers and decorative layers, and its perceived color remains unchanged over long periods, since it has weathering-resistant decorative layers serving as protective covering for the surfaces of the core.
The resin in HPL sheets has a twin function, which consists firstly in binding the wood fibers and/or cellulose fibers and secondly in filling the space between the fibers. Known solid sheets or HPL sheets use precondensed phenolic resins for binding the fibers and for filling the space between the fibers.
It is an object of the invention to improve a sheet of the type described at the outset in such a way that various physical properties can be achieved, and in particular moisture repellency is improved, without any excessive sacrifice of the sheet's mechanical properties.